The goal of this Roybal Center for Personalized Trials: Physical Activity Promotion to Foster Successful Aging is to generate intervention development research for promoting physical activity in older adults, and so launch several independent programs of research. We will accomplish these activities: 1. Comprehensively and creatively pilot-test a number of stage I-V behavior change interventions to increase exercise and decrease sedentary behavior 2. Develop and maintain an interdisciplinary Network on Behavior Change in Successful Aging for aiding in the rapid identification and refinement of the most innovative pilot physical activity studies 3. Contribute to the science of behavior change by testing behavior change theories, techniques, and mechanisms for promoting physical activity in older adults 4. Provide pilot project and future program of research support to under-represented investigators 5. Build an evidence-based, disseminable suite of personalized behavior change interventions and measures of mechanisms of action for improving the quality and quantity of older adults? lives. Aim of Pilot #1 is to efficiently identify the distinct behavior change techniques (i.e., building blocks) that are preferred and effective for improving low intensity physical activity (walking) in older, otherwise healthy, community-dwelling adults to be included in later-stage multi-behavior change technique interventions. Aim of Pilot #2 is to formally test the Habit Formation theory to improve walking in a second sample of older, otherwise healthy, community-dwelling adults, by employing its behavior change techniques and hypothesized mechanism of action?cognitive automaticity. If detected, heterogeneity in attaining a walking habit will inform later stage habit intervention studies. We will advance translational knowledge about preferences and effectiveness of behavior change techniques to promote physical activity in older adults. We will have a Network of international investigators who disseminate behavior change intervention development activities into their networks, and their programs of research. We will have conducted 18 pilot studies. We will have provided useful tools to create behavior change interventions and decide whether Personalized or N-of-1 methods are useful for Stage I intervention development for other physical activity interventions. We will provide a repository of useful behavior change techniques and mechanisms of action, so that we have a large impact upon physical activity interventions development with other older adult populations, and for other physical activity interventions. We will be successful if we have created practical, science-based, prevention-driven suite of behavioral interventions that are successfully implemented in the real world, to improve the health and vitality of older adults.